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The pear-shaped body of the adult is 0.3 -0.5 mm (0.01 - 0.02 inch) long, of , dark red to purple in color, and has numerous pinkish-white bristles („setae“).

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Citrus red spider mite

Adult citrus red spider mite

Biology

Damage

P. citri adults and nymphs puncture cells at the plant surface (up to 2-3 layers deep) with their piercing-sucking mouthparts, and ingest their contents. Their preferred feeding sites are the upper sides of young leaves, but fruits and green twigs may also be attacked. The resulting lesions appear as small grey marks („stippling“). When their number increases, these merge into spots that give whole areas of the leaf or green fruit a silvery appearance. The rind of more mature oranges or lemons will turn pale yellow. With ongoing infestation, the loss of photosynthetically-active tissue and (especially when the plants are under water stress) mesophyll collapse will lead to shedding of leaves („firing“) and fruits. In severe cases, whole twigs may die back. Yield may be significantly reduced. The inner quality of the remaining fruits is not adversely affected, and they are even larger in size due to a thinning effect. This will economically compensate only for a slight yield loss, however, and if the fruits are blemished, they will lose market value.

Lifecycle

Each adult female of P. citri lays 20-40 eggs, 2-3 per day, over a period of 2-4 weeks, preferably along the main veins of the leaves. After 1-4 weeks, the larvae hatch and spread over the plant. They begin feeding once they have moulted into the protonymph stage. Within a few days, they become deuteronymphs and finally, adults. The larvae have only six legs, whereas all other mobile stages have eight.

Before each molt, the nymph becomes inactive for some time and remains anchored to the plant surface only by its stylet. During this „chrysalis“ stage, the insect's old cuticle separates from the new one underneath and the body then shows an off-white tinge. Under optimal conditions (24 - 26°C; 75.2 - 78.8°F, 50 -70 % RH) the whole generation time may be as short as 12 days. The mite mortality rate increases under very hot and dry conditions as well as in continuously humid weather. Their development will slow down during the cool season, but will not cease completely. As soon as conditions improve, e.g. due to new plant growth or during a period of dry weather, the population may multiply within days. The adult mites can live several weeks.

Occurrence

Additional Crop Information

Agricultural Importance

The mites produce relatively little silk, but enough to allow them to be transported easily by the wind. They are also spread by insects and birds, or by humans and their tools. Like its host plant, P. citri probably originates from South-East Asia. It is nowadays found worldwide, wherever Citrus is grown. The Citrus red spider mite is an economically important pest, although the damage it causes varies strongly between regions as well as seasons. In some areas it is considered a minor issue, in others a serious threat. This might at least partly be explained by the crucially important - but too often ignored - influence of water supply on the severity of infestations by this mite.

Control

Useful non-chemical contribution to Integrated Weed Management

The damage caused by this mite is decisively influenced by irrigation. An adequate water supply considerably reduces the susceptibility of the trees to infestation.
The mite's enormous reproductive potential means that monitoring is very important. Use a hand lens to detect immature stages, too.
Avoid contact of fruits, e.g. on laden branches, with plants (grass, weeds) on the ground. P. citri has numerous natural enemies.

Chemical Control

If treatment is necessary, selective pesticides are preferable. Broad-spectrum insecticides may aggravate the situation.